If you are significantly smaller it can definitely kick start your economy. It can smooth out a lot of price and input issues. Most of your buildings will now just 'work', your general standing of living will go up, you'll have less budget issues.
The downside though is that you basically build up a huge dependency on that market. You are essentially a 'subject' of that other nation, and your power ranked will be capped because of that. It might not feel like the classical subject status, but you're a subject just the same. Yes, you can freely leave the union, you don't have to go to war or anything, but... unless you *really* prepared your economy for breaking out, you won't realize how much you depend on that market until you try to leave and your economy basically explodes (ask me how I know).